Many sitcoms are filmed live. That's the problem. A sitcom can take hours to film, because of multiple takes, scene changes, clothing changes, technical problems, and general breakdowns.

By the time an audience has sat through a punchline seven times over an hour, they're not going to laugh as loudly as they would as if it were the first time they heard it. All sitcoms are "sweetened" with added laughter during post-production. That's the only way to make it appear that the live show was filmed live.